Access to the Internet and the need to download and view vast quantities of data at greater and greater speeds, along with a frequent requirement for portability and a small footprint are placing greater demands on the capabilities of display devices. The display device of choice for such applications is a flat-panel display, but the current liquid crystal display (LCD) technology in use by most flat-panel displays is limited in its ability to meet these increasing demands. A new display technology, however, offers considerable promise for overcoming the limitations of the LCD technology. That new technology is based on the application of organic light-emitting devices (OLEDs), which make use of thin film materials that emit light when excited by an electric current.
The typical OLED 10, as shown in FIG. 1A, includes a multi-layer sandwich of a planar glass substrate 12, an anode layer 14 of Indium Tin Oxide (ITO), at least one organic layer 16, and a reflecting cathode 18. Typically, the glass substrate 12 has a thickness, tsub, of about 1 mm and an index of refraction, nsub, of about 1.5. The ITO layer 14 has a typical thickness, tITO, of about 100 nm and an index of refraction, nITO, of about 1.8. The organic layer 16 has a typical thickness, torg, of about 100 nm, and an index of refraction, norg, of between about 1.6 and 1.8. The cathode 18 is usually made of Mg:Ag or Li:Al. As shown, the device can include an electron transporting (ETL) and electroluminescent (EL) layer 16a of aluminum tris(8-hydroxyquinoline) (Alq3), with an index of refraction, nalq, of about 1.72. The device can include a hole transporting layer (HTL) 16b of 4, 4′-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl (α-NPD), with an index of refraction, nNPD, of about 1.78.
Organic light emitting devices (OLEDs) make use of thin film materials that emit light when excited by electric current. A representative organic emissive structure is referred to as the double heterostructure (DH) OLED, shown in FIG. 1B. In this device, a substrate layer of glass 110 is coated by a thin layer of a transparent, conductive oxide such as indium-tin-oxide (ITO) 111. Next, a thin (100-1000 Å) organic hole transporting layer (HTL) 112 is deposited on ITO layer 111. Deposited on the surface of HTL 112 is a thin (typically, 50-1000 Å) emission layer (EL) 113. The EL 113 provides the recombination site for electrons injected from a 100-1000 Å thick electron transporting layer 114 (ETL) with holes from the HTL 112. Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference.
The EL 113 is typically doped with a luminescent material to tune color of the OLED. The device as shown in FIG. 1B is completed by depositing metal contacts 115, 116 and top electrode 117. Contacts 115 and 116 are typically fabricated from indium or Ti:Pt:Au. Electrode 117 is often a dual layer structure consisting of an alloy such as Mg:Ag 117′ directly contacting the organic ETL 114, and a thick, high work function metal layer 117″ such as gold (Au) or silver (Ag) on the Mg:Ag. The thick metal 117″ is opaque. When proper bias voltage is applied between top electrode 117 and contacts 115 and 116, light emission occurs from emissive layer 113 through the glass substrate 110. In this case, the ETL 114 is typically doped with a luminescent material.
Another known organic emissive structure referred to as a single heterostructure (SH) is shown in FIG. 1C. The difference between this structure and the DH structure is that multifunctional layer 113′ serves as both EL and ETL. One limitation of an un-doped device is that the multifunctional layer 113′ must have good electron transport capability.
Yet another known LED device is shown in FIG. 1D, illustrating a typical cross sectional view of a single layer (polymer) OLED. As shown, the device includes a glass substrate 40 coated by a thin ITO layer 30. A thin organic layer 50 of spin-coated polymer, for example, is formed over ITO layer 30, and may provide all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 60 is formed over organic layer 50. The metal is typically Mg, Ca, Li or other conventionally used low work function metal.
An example of a multicolor electroluminescent image display device employing organic compounds for light emitting pixels is disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745. These patents disclose a plurality of light emitting pixels that contain an organic medium for emitting red, green, and/or blue light.
An important figure of merit for a display system is the efficiency of conversion of input power to emitted light. In OLED displays, a critical factor in determining this system efficiency is coupling efficiency (ηext) with which internally generated light is coupled out of the device. As shown in FIG. 1A, a large fraction of light generated in an OLED is never coupled out of the device (a) since it is waveguided in either the ITO electrode (c), the organic layers, or the transparent substrate (b). This waveguided light is either absorbed in the structure of the device or coupled out of the edges of the device. In order to meet expected demands of future display systems, there is a need to improve the coupling efficiency of OLEDs.